1. Field of the Invention
The present invention relates to piezoelectric/electrostrictive devices which are applied to optical communication devices such as an optical switch, optical add-drop multiplexers, and variable optical attenuator, image display apparatuses such as a display and projector, mechanical driving apparatuses such as a micro-pump, droplet discharge apparatus, and precision gas flow rate control apparatus for a semiconductor manufacturing apparatus, or various types of sensors, and to a method for manufacturing the piezoelectric/electrostrictive device.
2. Description of the Related Art
In recent years, in fields of optics, precision machines, semiconductor manufacturing, and the like, there has been a demand for a displacement control element for adjusting an optical path length or position on the order of sub-microns. To meet the demand, development of piezoelectric/electrostrictive devices such as an actuator and sensor has been advanced, in which a strain caused in applying an electric field to a ferroelectric or antiferroelectric member is used based on an inverse piezoelectric effect or an electrostrictive effect. For the displacement control element using an electric field inductive strain, as compared with a conventional electromagnetic system by a servo motor, pulse motor, and the like, there are characteristics that micro displacement control is easy, mechanical/electrical energy conversion factor is high and power saving is achieved, and the elements can be mounted with super precision and can contribute to miniaturizing/lightening of products. It is considered that a field of application is steadily increasing.
For example, it has been proposed that the piezoelectric/electrostrictive device be used as an actuator for switching a transmission path of an input light (see WIPO Pamphlet WO 02/52327 A1). One example of the optical switch is shown in FIGS. 2(a), 2(b). An optical switch 200 shown in FIGS. 2(a), 2(b) includes an optical transmission section 201, optical path change section 208, and actuator section 211. In detail, the optical transmission section 201 includes an optical reflective surface 101 disposed in a part of a surface disposed opposite to the optical path change section 208, and optical transmission paths 202, 204, 205 disposed toward three directions starting from the optical reflective surface 101.
Moreover, the optical path change section 208 is disposed in the vicinity of the optical reflective surface 101 of the optical transmission section 201 in a movable state, and includes an optical introductory member 209 formed of a translucent property, and an optical reflective member 210 for totally reflecting light. Furthermore, the actuator section 211 is displaced in response to an external signal, and includes a mechanism for transmitting the displacement to the optical path change section 208.
For the optical switch 200, as shown in FIG. 2(a), the actuator section 211 operates in response to the external signals such as application of voltage. By the displacement of the actuator section 211, the optical path change section 208 is detached from the optical transmission section 201. A light 221 inputted in the optical transmission path 202 of the optical transmission section 201 is totally reflected without being transmitted in the optical reflective surface 101 of the optical transmission section 201 in which a refractive index is adjusted to a predetermined value, and transmitted to the optical transmission path 204 on an output side.
On the other hand, conversely, when the actuator section 211 is brought into an inoperative state from this state, as shown in FIG. 2(b), the displacement of the actuator section 211 returns to an original state, and the optical introductory member 209 of the optical path change section 208 contacts the optical transmission section 201 in a distance which is not more than wavelength of the light. Therefore, the light 221 inputted in the optical transmission path 202 is brought to the optical introductory member 209 from the optical transmission section 201 by the optical introductory member 209, and is transmitted in the optical introductory member 209. The light 221 transmitted in the optical introductory member 209 reaches the optical reflective member 210, and is reflected by a reflective surface 102 of the optical reflective member 210. Accordingly, the light is transmitted to the optical transmission path 205 on an output side, different from the layer reflected by the optical reflective surface 101 of the optical transmission section 201.
Loss in switching is reduced in the above-described optical switch, and there has been a demand for an actuator in which large displacement amount and high generation force can be realized. However, the piezoelectric/electrostrictive device in which a plurality of uni-morph or bi-morph piezoelectric/electrostrictive elements heretofore known are arranged in a plane is based on a bending mode, and it has therefore been difficult to simultaneously satisfy a displacement amount and generative force.
Moreover, from this time, with an advance of construction of an optical network system which does not carry out optical/electric conversion, the number of circuits of a photonic router increases. On the other hand, the photonic router has been requested to be further miniaturized. Therefore, it has been requested that more optical switches be disposed as constituting elements of the photonic router per a certain area. However, the conventional piezoelectric/electrostrictive device includes a constitution in which the major surface of the piezoelectric/electrostrictive member is disposed substantially vertical to a bending displacement direction. Therefore, a device dimension (width or thickness) itself cannot help getting large in order to increase a generated displacement amount, and it is difficult to reduce a pitch and to arrange the elements in a high density. In the present specification, the term “major surface” means a surface showing an intended function as a device, a member or an element.
Additionally, with respect to high-density arrangement of the piezoelectric/electrostrictive elements, a proposal has heretofore been made in Japanese Patent No. 3058143. It is disclosed that an actuator shown in FIG. 1 of Japanese Patent No. 3058143 is a piezoelectric actuator optimum for an ink jet system recording apparatus and that pillar piezoelectric elements functioning as a driving mechanism are two-dimensionally arranged in a checkerboard form based on a piezoelectric transverse effect. Moreover, the piezoelectric actuator is assumed to have an effect that the number of ink jet nozzles per unit area can be increased in the recording apparatus of the ink jet system.
However, for the disclosed piezoelectric actuator, green sheets coated beforehand with common electrodes or application electrodes are stacked and fired, and thereafter a dicing saw is used to process a trench for separating/isolating the pillar piezoelectric elements. Therefore, there have been at least the following problems.
One of the problems is generation of particles. It has been considered that the particles are easily generated especially in a surface mechanically processed by the dicing saw. For a reason for this, the piezoelectric member including ceramic particles is processed as described above, and the probability of the presence of the transgranularly fractured crystal grains or the probability of the generation of cracks increases in the processed surface, that is, the whole surface constituting the piezoelectric element. As a result, the particles are easily detached by a repeated operation of the piezoelectric element over a long period.
When this piezoelectric actuator is used in an actuator section of the optical switch described above, the generated particles stick to the optical reflective surface of the optical transmission section, and propagation loss is caused in an unexpected portion by the reflection. There is also a possibility that the particles enter a gap between the optical change section and the optical reflective surface, and a trouble is caused in a contact operation of the optical path change section. These can be factors for unsteadily changing the optical path, and are not preferable.
A second problem is limit to arrangement density. The piezoelectric actuator includes a structure in which the electrode is contained beforehand in the piezoelectric element. Therefore, the structure is influenced by distortion at the time of the firing, a layer structure including the electrode and piezoelectric member of each of the separated/isolated piezoelectric element easily becomes nonuniform, and fluctuations of characteristics are caused among the elements. Moreover, in consideration of the firing distortion, the size (width or thickness) of the element itself cannot but increase. Therefore, even if the limit of the arrangement density is higher than that of the piezoelectric/electrostrictive device including a plurality of uni-morph or bi-morph type piezoelectric/electrostrictive elements heretofore known and two-dimensionally arranged, it is difficult to reduce the pitch and to arrange the elements in a higher density.
In the example of the piezoelectric actuator disclosed in Japanese Patent No. 3058143, the piezoelectric element has a width of 0.3 mm, the trench has a width of 0.209 to 0.718 mm, and a density is such that one piezoelectric element is substantially disposed per 1 mm2. However, this cannot be said to be a sufficiently high density for meeting resolution required for an ink jet printer these days. This arrangement density cannot be satisfactory even in the optical switch in the example shown in FIGS. 2(a), 2(b).
A third problem is a limit of a displacement amount. In the disclosed piezoelectric actuator, the separated/isolated piezoelectric elements are formed by dicing saw processing. However, by restrictions in the processing, when the depth of the trench, that is, the height of the piezoelectric element increases, straightness of the structure easily deviates. When the piezoelectric devices are arranged in a high density, the distance between the piezoelectric elements disposed adjacent to each other is reduced, the deviation of the position accuracy of the piezoelectric element relatively increases. Therefore, the height of the piezoelectric element has to be necessarily limited to be small for the arrangement in the high density.
For the transverse effect type element in which the generated displacement amount depends on the height of the piezoelectric element, the obtained displacement amount is not sufficient with the limit to the height. Therefore, even when the generated displacement amount of the disclosed piezoelectric actuator is larger than that of the piezoelectric/electrostrictive device including the plurality of heretofore known uni-morph or bi-morph type piezoelectric/electrostrictive two-dimensionally arranged, the amount has not been satisfactory for the actuator of the ink jet system recording apparatus (ink jet printer) or the optical switch in recent years.